1. Field of the Invention
The present invention relates to a movable body apparatus, such as a resonance-type movable body apparatus, including a vibratory system with a movable body, an optical deflector using the movable body apparatus, and an optical instrument using the optical deflector. Particularly, the present invention relates to a movable body apparatus that advantageously controls the driving frequency of a drive signal for driving a movable body. The optical deflector using the movable body apparatus can be preferably used in optical instruments, such as image displaying apparatuses like a scanning-type display, and electrophotographic image forming apparatuses like a laser beam printer (LBP) and a digital copying machine.
2. Description of the Related Art
Conventionally, a rotary polygonal mirror is used as an optical deflector in an image forming apparatus. In recent years, an optical deflector with a mirror capable of vibration in a resonance manner is proposed for the purpose of replacing the rotary polygonal mirror thereby. Such an optical deflector has advantages as follows. Compared to an optical deflector using the rotary polygonal mirror, the size can be greatly reduced. The consumption electrical power also can be reduced. There theoretically exists no problem of so-called face tangle. In particular, with such an optical deflector formed of a Si single crystal capable of being fabricated by a semiconductor processing method, no metal fatigue exists theoretically, and the endurance property is typically excellent.
FIG. 9A illustrates the construction of a driving circuit for an optical deflector generally usable in an image forming apparatus using the resonance-type optical deflector. Japanese Patent Application Laid-Open No. 2006-221030 A discloses such a driver. In this construction, which is illustrated in FIG. 9A, a light beam from a light beam generator 23 is deflected by a deflecting mirror 20, and the light beam is scanned in a main scanning direction that is a longitudinal axial direction of a photosensitive body 24. A drive signal generating circuit 21 applies a drive signal of voltage across a coil 22 to generate a magnetic field. The magnetic field interacts with a magnetic field from a magnet arranged on the deflecting mirror 20. Thus, a torque acts on and vibrates the deflecting mirror 20. The amplitude of the vibration of the deflecting mirror 20 gradually increases at a drive start time. A portion of the photosensitive body 24 is exposed by the light beam scanned in the main scanning direction, and this exposure is performed according to an image whose printing is requested.
Beam detectors 25 and 26 (simply referred to as BD in this specification) are disposed on both sides of the photosensitive body 24. From a time (light beam detection time) at which the light beam impinges on the BDs 25 and 26, an angular displacement measuring circuit 28 can measure a deflection amount of the light beam (i.e., the amount of an angular displacement of the deflecting mirror 20). An image forming circuit 27 controls the light beam generating circuit 23, based on a signal from the angular displacement measuring circuit 28 and data stored in an image memory 200 through a communication interface (I/F) 201.
With respect to the drive signal generated by the above-described drive signal generating circuit 21, driving can be executed most efficiently by a drive signal with a resonance frequency, which the deflecting mirror 20 has as its property. A desired angular displacement of the deflecting mirror 20 can also be obtained. The resonance frequency, however, has a temperature-dependent property as illustrated in FIG. 9B. Accordingly, to generate a drive signal with a resonance frequency, it is necessary to measure the relationship between information of the angular displacement of the deflecting mirror 20 and the drive signal.
As the measuring method, there exists a method of measuring the relationship based on a change in phase difference between a phase of the drive signal and a phase of the angular displacement signal (see Japanese Patent Application Laid-Open No. 2002-78368 A). In other words, a resonance frequency detecting circuit 203 detects the phase difference between the drive signal generated by the drive signal generating circuit 21 and the amount of the angular displacement detected by the angular displacement measuring circuit 28, and the circuit 203 obtains a resonance frequency based on a change in the phase difference.
In the above-described construction, at a drive start time, such as a time when an electrical power source is switched on, a system controller 202 supplies drive start signals to the drive signal generating circuit 21, the image forming circuit 27, and a storing portion 29, respectively. At this moment, the drive signal generating circuit 21 generates a drive signal, with an initial frequency stored in the storing portion 29, as a preset value. After the drive start, the drive signal generating circuit 21 alters the frequency of the drive signal within a variable range of the resonance frequency until signals are input into the BDs 25 and 26, so that the angular displacement of the deflecting mirror 20 can be obtained.
FIG. 10 illustrates a manner how of the drive control of the deflecting minor 20 is performed at the drive start time. The drive control of the mirror 20 starts at a preset frequency ffix. After that, the driving frequency is linearly changed according to a time-changing function α(t). When the angular displacement amplitude of the mirror 20 is increased to a magnitude at which the signal can be input into the BD, a feedback drive control at a steady operating time begins to be executed while the resonance frequency is measured by the above-described method. FIG. 10 indicates a time T1 of a period from the drive start time to the BD signal input time.
According to the above-described technology, in which the drive control is started with a drive signal with the frequency preset at the drive start time, the following disadvantageous phenomenon can occur. If the preset frequency is largely away from a resonance frequency of the deflecting mirror 20 at the drive start time, the angular displacement amplitude of the mirror 20 cannot increase sufficiently fast enough, even if the amplitude of the drive signal is increased. To increase the angular displacement amplitude, the driving frequency of the drive signal needs to be brought close to a resonance frequency at that time, as described above. In the above case, however, it takes a considerable amount of time to bring the driving frequency to a frequency at which the angular displacement amplitude is well increased. Thus, it is difficult to quickly start a steady state operation, such as a first printing.